"This is a warning light that there's something going on here, and school districts should look into it," says Jack Jennings, president of the Center on Education Policy in Washington. But noting that long-term trends are still positive, he says, "it's not an alarm bell."

Particularly surprising is the fact that the plateau is in elementary school – in general, the most reliable source of good news for education gains. And it occurred across the board: There was no change from 2007 for any racial or income group; no change in any achievement gap; and no change in the percentage of students at any of the three achievement levels (basic, proficient, and advanced).

On average, fourth-graders scored 240 points on the exam – up from 213 points in 1990 but unchanged from 2007. Thirty-nine percent scored at or above the "proficient" level – the target benchmark where students should be.

The gap between the scores of black and white fourth-graders was 26 points – down from a 32-point gap in 1990, but unchanged from 2007.

Most observers, like Mr. Jennings, caution against reading too much into a single data point: It may indicate an interruption of the steady growth trend, rather than a reversal of it. But the lack of progress, education advocates also say, is striking enough that the country should be asking hard questions about its math education, particularly for younger students.

"This may mean we've gotten all the octane we can out of our current math teaching force," says Amy Wilkins, a vice president at the Education Trust, a nonprofit dedicated to reducing achievement gaps. "What can we do to get more really good math teachers not only into the profession, but into high-poverty and high-minority schools?"

In middle school, she notes – where overall scores improved slightly but the achievement gap remained the same – students of color are much less likely than other kids to be taught math by teachers with a math background. "We're not going to close the achievement gap until we close the teacher talent gap," she says.

David Driscoll, former Massachusetts commissioner of education, also focused on teaching in his remarks at the NAEP announcement Wednesday. In Massachusetts, he noted, elementary-school teachers used to be able to get a license without answering a single math question correctly on the state exam. He worked to increase the rigor, and 55 percent of test takers failed the new exam this year, he said.

"If we are to succeed, we must all work together to provide comprehensive, challenging math courses for future educators," Mr. Driscoll said.

While the plateau for fourth-graders doesn't undo the significant gains made during the past two decades, it's also true that the gains in the past six years have been slowing. This is yet another sign, according to some educators, that it may be tougher in the future to log steady improvements or to reach the 2014 goal of 100 percent proficiency for all students, which was set by the No Child Left Behind Act.

"We've done all the easy stuff," says Ms. Wilkins. "We've been doing incremental changes that lead to incremental improvement.... This may be the signal to say, 'OK, it's time to really kick in.' "

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Another report card on achievement gaps

Earlier this month, the Center on Education Policy released its findings on student test scores in all 50 states since 2002, when the No Child Left Behind Act took effect. Click here for the conclusions.